Neurologic
imaging

Summary

MRI provides images that are reflective of regional differences in proton
concentrations and the physico-chemical environment of protons (e.g. water
protons show different MR behavior from protons in larger molecules (lipids,
proteins, etc.); protons in hydration water show different MR behavior than
protons in unbound water). In particular, gray matter can be distinguished
from white matter and CSF (cerebro-spinal fluid) by those proton characteristics.

Since
pathological changes in tissue composition can alter proton distribution,
tumoral infiltration, edema, and bleeding can be distinguished from the surrounding
healthy tissues. Three main applications (schizophrenia, multiple sclerosis,
Alzheimer' s Disease and neurosurgical guidance) are the paradigms for three
basic approaches to the extraction of information: quantitative manual, quantitative
automated, and qualitative. These approaches are driven by the capability
of MRI to characterize tissues.

Alzheimer's Disease

Several
MRI-based studies of age-related volumetric changes in the gray and white
matter of healthy human volunteers have shown that atrophy is associated
with normal aging and can be recognized as a relative increase in intracranial
fluid compartments on MR images. Recent studies have supported this finding.
MRI studies focusing on mildly impaired AD patients have uniformly
reported significant and striking differences between MRI measures of the
medial temporal lobe in AD patients and controls. Thus, with the help
of MRI, volume measurement has been reported as an excellent means of differentiating AD patients
from controls.

Multiple Sclerosis

Conventional
spin echo (CSE) MRI, has been the gold standard for imaging of MS since
its sensitivity to white matter abnormalities was first identified in the
early 1980s. Active lesions were detected by intravenously-administered paramagnetic
contrast agents in regions with a compromised blood-brain-barrier. The different
histological components of a lesion modulated MRI signal intensity to different
extents.
Several studies, including a preliminary analysis of a longitudinal MR study
of the natural course of MS with automated methods indicated that changes
in MR-based lesion load measurements are significantly correlated to changes
in clinical outcome measures, such as the expanded disability status scale (EDSS)
and the number or frequency of attacks.